Acid sulfite pulping



ACID SULFITE PULPING Stewart E. Seaman, White Plains, N. Y.

No Drawing. Application October 25, 1951, Serial No. 253,205

3 Claims. (Cl. 92-11) This invention relates to the manufacture of wood pulp and has for its purpose the procurement of a high yield of high grade pulp with a saving in operating costs and time.

The invention is applicable to the processes employed in the production of sulphite pulp and resides in a combination of steps which cooperate to effect the desired purpose.

The process of this invention results in the production of pulp having a higher percentage of alpha cellulose than is obtained by conventional processes. It also results in a considerable saving in time and in substantial saving in the cooking liquor.

One of the salient features of the invention is the fact that it enables the use of relatively high concentration of the cooking medium and high temperatures without degradation of the cellulose and consequently materially shortens the cooking time. Thorough wetting of the chips with the cooking liquor is an essential preliminary to such cooking and the invention contemplates special procedure for wetting to cause thorough and rapid penetration of the liquor into the chips and the elimination of air and other gases which would block such penetration and also deleteriously affect the cellulose and the yield.

With complete penetration and Wetting of the chips, it has been found that the cooking may be done in the absence of free cooking liquor in a hot vapor atmosphere and that the temperature of such atmosphere may be higher than the usual permissible temperatures of cooking liquors without injury to the fiber of the pulp.

With this quick cooking process in the absence of free liquor, it is found that the liquor which is caused to penetration the chips may be and preferably should be of higher concentration than is possible in the processes where the chips are covered with free liquor during cooking.

In applying the process, the cooking liquor will have as much as 2% of the bisulphite and as much sulphur dioxide S02 as the liquor will retain, 3 /2% or more of sulphurous acid. After filling the digester with chips, a vacuum is drawn until the chips are substantially devoid of gas. Then hot cooking liquor at preferably from 85 to 100 C. and having approximately 2% of bisulphite and as much free S02 as possible is pumped into the digester in quantity sufiicient to cover the chips and fill the digester. Additional liquor is then pumped in under hydraulic pressure, preferably the maximum pressure that the digester will tolerate, usually around 100 pounds or higher, and this pressure is maintained for a sufficient length of time to effect complete penetration of the chips.

Then all free liquor is drawn off from the digester and steam under pressure is admited and the chips are cooked in this vapor atmosphere. A proper cooking temperature is around 170 or even higher and this tem-- perature and pressure are maintained until cooking is complete, provision being made to trap away all free liquor as the steam condenses. Then the digester is discharged.

It is found that the total time for the cycle in this process is on the average around three hours, as follows: to minutes for filling the digester; 30 minutes for evacuating the gases from the chips; one-half to one hour Patented Pee. g8,

2 i. for hydraulic impregnationgone hour :forcooking; onequarter to one hour for discharging. i

The removal of the air from the wood structure as a preliminary to penetration is desirable for the reason that the air would be trapped and compressed in the interstices and would exclude the liquor and furthermore because air acts deleteriously on the cellulose and on the yield of cellulose. With the air excluded and with the hydraulic pressure to which the liquor is subjected, the wetting takes place rapidly and the complete impregnation is accomplished in much less time than is required in the procedure heretofore employed for the purpose.

The advantage of drawing off all free liquor at the end of the wetting operation is apparent. The temperature having been below cooking temperature the liquor is clean when drawn off and is usable for the next digester batch. When the cooking is completed and the digester is emptied there is minimum cooking liquor to discharge into sewers and pollute streams.

In the bisulphite process the cooking is done by the sulphurous acid absorbed within and wetting the chips and the organic acids which are produced during cooking. The higher than ordinary percentage of bisulphite is used for the reason that the percentage of total liquor in the digester is considerably less than ordinary, one and onehalf to one as compared with from six to ten to one, and the bisulphite concentration needs therefore to be higher to provide the alkalinity necessary to neutralize the strong organic acids produced during cooking.

In addition to saving cooking liquor this process also effects a substantial saving in steam since it requires only from one-fourth to one-sixth of the amount of steam needed in the conventional processes to heat cooking liquor to cooking temperature. Cooking proceeds rapidly and uniformly since the hot steam enters directly into intimate contact with the cooking liquor in the chip structure and the heat distribution does not depend upon the circulation of a volume of liquid. The virtually double concentration of the bisulphite as compared with usual processes suffices to neutralize the stronger organic acids while the cooking proceeds rapidly due to the presence of the maximum amount of S02 and the rapid formation of organic acids under the heat and atmospheric conditions. By such a process it is found that a higher yield of cellulose fiber is obtained than is obtained from processes in which the chips are submerged in cooking liquor and that there is a higher percentage of alpha cellulose obtained than is possible with such other processes. For example, it is found this process yields around 91% alpha cellulose as compared with a maximum of 89% and usually from to 88% by the submerged cooking processes and that there is a yield of approximately 48% of raw unbleached pulp as compared with 42 to 44% with the usual processes.

It will be understood that this process is particularly suited for steel digesters. Where the digesters are lined with brick, as is the case with most sulphite mills, it will not be feasible to draw a high degree of vacuum with a new batch immediately after discharging the digester of a previous cooking since the residual heat would cause the liquor which is in the porous bricks to flash into steam and do possible damage to the lining.

While specific values have been given in the foregoing description of certain particular examples of the process of the invention, it will be understood the invention comprehends ranges of values which will effect substantially the same result within the intent and principle of the invention as defined in the following claims.

What is claimed is:

l. A process for the production of sulphite pulp consisting in introducing chips into a digester, filling the digester with a cooking liquor having approximately 2% of bisulphite expressed as S02 and not less than 3% of HzSOs expressed as S02 and applying hydraulic pressure thereto and maintaining the pressure until the chips are completely impregnated with the liquor, withdrawing r, .3 4 all f(r1ee liquor grom the digester and admitting steamf into References Cited in the file of this patent e igester an maintaining a vapor temperature 0 approximately 170 C. until the chips are cooked. UNITED STATES PATENTS 2. The process for production of sulphite pulp as Number Name Date defined in claim 1 together with the step of evacuating 5 1,859,847 Rue et al May 24, 1932 the digester before introducing cooking liquor and re- 1,877,899 Bradley et a1 Nov. 15, 1932 moving entrapped air from the chips. 1,903,962 Dreyfus Apr. 18, 1933 3. The process of claim 2 in which the cooking tem- 2,229,886 Dunbar Jan. 28, 1941 perature' is maintained at approximately 170 for ap- 2,599,572 Miller June 10, 1952 proximately one hour. 10 

1. A PROCESS FOR THE PRODUCTION OF SULPHITE PULP CONSISTING IN INTRODUCING CHIPS INTO A DIGESTER, FILLING THE DIGESTER WITH A COOKING LIQUOR HAVING APPROXIMATELY 2% OF BISULPHITE EXPRESSED AS SO2 AND NOT LESS THAN 3% OF H2SO3 EXPRESSED AS SO2 AND APPLYING HYDRAULIC PRESSURE THERETO AND MAINTAINING THE PRESSURE UNTIL THE CHIPS ARE COMPLETELY IMPREGNATED WITH THE LIQUOR, WITHDRAWING ALL FREE LIQUOR FROM THE DIGESTER AND ADMITTING STEAM INTO THE DIGESTER AND MAINTAINING A VAPOR TEMPERATURE OF APPROXIMATELY 170* C. UNTIL THE CHIP ARE COOKED. 